The effects of external calcium on long-term potentiation in the rat hippocampal slice.

Extracellular excitatory postsynaptic potentials (EPSPs) were recorded from the stratum radiatum of CA1 of the rat hippocampal slice. The effect of altering the external Ca concentration was studied on the amplitude of the low frequency evoked EPSP preceding and following the production of long-term potentiation (LTP). The double logarithmic plot of the relationship between the EPSP amplitude and external Ca had a maximum slope of 2.1, implying Ca cooperativity in transmitter release. The production of LTP did not alter the slope. The amplitude of LTP was found to be highly dependent on the external Ca concentration, with LTP increasing from a 5% increase in the EPSP amplitude in 0.8 mM Ca, to a 65% increase in the EPSP amplitude in 2.0 mM Ca.